High-performance radio frequency (RF) photonic links, e.g., high dynamic range analog photonic links, are key to the operation of many RF systems. Historically, these analog photonic links, which operate with a high spurious free dynamic range (SFDR), are built from three major components, typically separated by optical fibers; a laser, a modulator, and a photodetector. In a high-SFDR system, the laser provides high optical power with very low noise, in particular the laser RIN. The laser is typically packaged in its own hermetically sealed module that includes a discrete optical isolator between the laser and its output fiber, which significantly reduces the level of any optical reflections back into the laser; this is required because lasers are very sensitive to optical reflections, and their noise level, both RIN and linewidth (where linewidth refers to both the low frequency phase noise and the Lorentzian linewidth of the laser), are significantly increased if optical reflections return back into the laser. A schematic of a Prior Art RF photonic link made from such discrete components is shown in FIG. 1.
The laser provides a single frequency optical carrier signal, which is fed into the modulator through an optical fiber. An electrical input signal drives the modulator to provide a modulated optical signal at the modulator optical output; e.g. for intensity modulation using a Mach-Zehnder interferometer (MZI) based modulator, this will be a standard double sideband signal, including a carrier plus two sidebands. Other modulator types and modulation formats provide similar modulated spectra, a carrier (or suppressed carrier) plus one or more sidebands. The output of the modulator is fed into an optical fiber, which is used to connect the transmitter side of the system to the receiver side of the system—this may be close, within a single unit, or between units, or may be longer distances up to multiple kilometers. For long distances, such as 10's of km or more, additional optical amplification and filtering may be included. At the receiver side of the system, the fiber is coupled into a photodetector, which converts the optical signal into an output electrical signal.
In some cases both outputs of an MZI modulator are passed along a pair of optical fibers, and the two fiber outputs are detected using a pair of balanced photodetectors, to improve the system performance.
In some cases there is no optical fiber used to connect the transmitter and receiver, and the optical transmission is carried out within some other medium, such as a free space by using lensing to aim the transmitted light beam and receive that light beam, or by other waveguides or ways of routing the transmitted light to the receiver.
The operation of a high-SFDR analog optical link, e.g., SFDR≥110 dB·Hz2/3, is strongly dependent on the power level and the RIN of the source laser, which itself depends on having a high performance and broadband optical isolator to significantly reduce the level of any optical reflections returning to the laser. Discrete optical isolators, providing high isolation (>30 dB), low loss (<1 dB), and broad bandwidth (>30 nm), i.e., enough bandwidth to support lasers operating at a range of wavelengths that are modulated at high speed, are widely available to be used within laser modules. With sufficient optical isolation provided by the isolator, e.g., 30 dB, the increase in intrinsic RIN or linewidth of the laser is negligible for any small optical reflections returning to the laser module. The system itself is also designed to provide only very small reflections back from any point to the laser source.
Such systems require a high performance RF photonic transmitter, that includes a laser source and modulator. The laser operation is very sensitive to optical reflections from within the transmitter PIC or in the following optical system, which can increase the intrinsic laser relative intensity noise (RIN) and laser phase noise/linewidth, reducing system performance. This invention includes multiple approaches to reduce the level of optical reflections back to a source laser.